However, analysis of pupils’ attainment, engagement and wellbeing at school and their future aspirations by the UCL Institute of Education found that attending a grammar school had “no positive impact” on pupils’ self-esteem, attitude to school, future aspirations or vocabulary at age 14.

The study analysed data from 883 pupils in England and 733 in Northern Ireland who had similar academic achievements at primary school and came from families with similar incomes and education levels. All the pupils selected were from the Millennium Cohort Study, which follows the lives of around 19,000 children born in the UK between 2000 and 2001.

Researchers compared the results of tests children had taken in English, maths and verbal and non-verbal reasoning at ages three, five, seven and 11 as well as a vocabulary test at age 14. Social and emotional outcomes were based on answers given to a series of questionnaires at ages 11 and 14 about mental health, engagement at school, well-being and interaction with peers.

Professor John Jerrim, the study’s lead author, said the findings “suggest that the money the government is planning to spend on grammar school expansion is unlikely to bring benefits for young people”.

“Even those children who are likely to fill these new places are unlikely to be happier, more engaged at school or have higher levels of academic achievement by the end of year 9,” he said.

Instead, the government would be “better off directing their money towards areas of existing need”, according to co-author Sam Sims.

The study’s findings make it “increasingly difficult to understand the government’s rationale for spending money or expanding selective education rather than on improving education for all young people”, said John Hillman, director of education at the Nuffield Foundation, which funded the research.

A spokesperson for the Department for Education insisted that grammar schools “provide an excellent education” and pupils from disadvantaged backgrounds “achieve better results in selective schools”.

“We want more children from all backgrounds to have access to a world-class education, which is why all selective schools applying for funding to expand must not only be ‘good’ or ‘outstanding’, but must also make clear how they will increase their intake of disadvantaged pupils and work with local non-selective schools to improve outcomes for pupils of all backgrounds.”

But Dr Mary Bousted, the joint general secretary of the National Education Union, believes this is “not an appropriate use of public funds at a time when schools across the country are having to reach down the back of the sofa for spare change”.

Angela Rayner, Labour’s shadow education secretary, said the report was “simply the latest evidence that the prime minister’s pet project of grammar school expansion will fail even on its own terms”.

“Instead of pushing ahead with this discredited and divisive policy, the Conservatives should give every child the support they need, by reversing their cuts to school budgets and giving our teachers the pay rise they deserve.”